ok, ill give you a way to draw the same current using 30 leds... Use 1 resistor per led, each resistor led branch will draw around 25mA, so 30 will draw 0.75A since a 21 watt bulb draws 1.75A we are short an amp, so to draw 1 A... we could put a single resistor in parallel will all the resistor and leds, but that would mean we would need a 12 ohm resistor, further more it would need to be able to handle 12 Watts, quite a bit to use smaller resistors you could put a bunch in parallel, I will assume you are using 1/2 watt resistors.... since P = V(I) and they are in parallel with 12V (assuming the lowest ~12V), V = 12, P= .5... I = (0.5) / (12) = 42mA, therefore maximum these resistors can only handle 42mA, we will assume 40mA so the resistors we need will be 1/2 watt, and must draw 40mA each, since we need to draw 1A we will need.. 1A / 40mA = 25 quite a bit of resistors... now to find the resistance value needed, V = I * R, R = V / I therefore R = 12 / 40mA = 300, so you will need 25, 300 ohm resistors, you can check since the parallel resistance will be 300 / 12 = 12 ohm, so together it is 12ohm and each resistor will be burning off 0.48 Watts... If you use 1W resistors you can use fewer (12 @ 150 ohms) hope that helped... |